If you want to make perfectly poached salmon that isn't overcooked, drop it in cold water and gradually crank up the heat. Photo by Vicky Wasik.
High heat, like simmering or boiling water, will cause salmon's delicate proteins to tighten and toughen as soon as you lower it into your poaching water. That's why Daniel Gritzer at Serious Eats suggests you start salmon off in cold poaching water and slowly introduce heat until the liquid reaches 75C and your fish reaches a safe internal temperature between 45C and 60C. When your salmon is done cooking, let it rest for five minutes before serving. You don't even need to pre-make the court bouillon if you don't want to because you can easily add lemon juice and a few aromatics to the water while it heats up.
The Best Way to Poach Salmon [Serious Eats]